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Abstract:

The invention relates to a sensor assembly. The assembly includes a
sensor body 2 of appropriate construction (preferably substantially
ceramic) with a radial flange 8. A housing 20 is of two-part integral
construction and includes an annular groove or recess in which the radial
flange 8 of the sensor body 2 is received when the sensor assembly is in
its assembled form. The annular groove is defined by a pair of facing
shoulders 28, 36 each having an annular surface 30, 38 and a
substantially cylindrical surface 32, 40. The annular surfaces 30, 38 are
in sliding contact with the flange 8 and apply a compressive load to the
flange to form a hermetic seal between the housing 20 and the sensor body
2. The hermetic seal is maintained even if the sensor assembly is used at
high operating temperatures.

Claims:

1. A sensor assembly comprising: a sensor body having a radial flange;
and a housing having an annular groove in which the radial flange of the
sensor body is received, the annular groove being defined by a pair of
facing shoulders each having an annular surface and a substantially
cylindrical surface; wherein the annular surfaces of the shoulders are in
sliding contact with annular surfaces of the flange and apply a
compressive load to the flange.

2. The sensor assembly of claim 1, wherein the sensor body is formed
substantially from a ceramic material.

3. The sensor assembly of claim 1, wherein the sensor body includes one
or more electrically conductive layers and one or more electrically
non-conductive layers.

4. The sensor assembly of claim 1, wherein the housing is a two-part
housing.

5. The sensor assembly of claim 4, wherein one of the facing shoulders is
formed in a first housing part and the other of the facing shoulders is
formed in a second housing part.

6. The sensor assembly of claim 5, wherein the first housing part is
secured to the second housing part.

7. The sensor assembly of claim 5, wherein the first housing part is
brazed to the second housing part by a braze material.

8. The sensor assembly of claim 1, wherein the sensor body is not fixed
to the housing.

9. The sensor assembly of claim 1, wherein the sliding contact between
the housing and the annular surfaces of the flange under the compressive
load provides a hermetic seal between the housing and the sensor body.

10. A method of manufacturing a sensor assembly comprising the steps of:
providing a sensor body having a radial flange; locating the sensor body
in a two-part housing having an annular groove in which the radial flange
of the sensor body is received, the annular groove being defined by a
pair of facing shoulders each having an annular surface and a
substantially cylindrical surface, one shoulder being formed in a first
housing part and another shoulder being formed in a second housing part;
bringing the annular surfaces of the shoulders into contact with the
annular surfaces of the flange; and brazing the first and second housing
parts together to form an integral two-part housing by (i) raising the
sensor assembly to a particular brazing temperature during which the
first and second housing parts undergo thermal expansion, (ii) applying a
braze material to the first and second housing parts in a molten state,
and (iii) reducing the temperature of the sensor assembly so that the
braze material solidifies to secure the first and second housing parts
together to form the integral two-part housing and during which the first
and second housing parts undergo thermal contraction to apply a
compressive load to the flange.

11. The method of making a sensor assembly of claim 10, wherein the
annular surfaces of the shoulders are brought into contact with the
annular surfaces of the flange by applying a loading which forces the
first and second housing parts towards each other.

Description:

TECHNICAL FIELD

[0001] The present invention relates to sensor assemblies, and in
particular to sensor assemblies that incorporate a ceramic body and can
be used in high temperature operating environments.

[0003] A further range of suitable products include electromagnetically
transparent windows which are used to protect electromagnetically-based
systems from the high temperature and pressure experienced in a gas
turbine environment, for example. These windows can be made from
materials such as sapphire, quartz and diamond, as well as more
conventional ceramic materials. The term "transparent" is intended to
refer to any materials which do not impede the path of the
electromagnetic radiation to any significant degree. Such materials are
usually chosen for use with specific measurement systems which
incorporate optical, microwave or infra-red technology, for example.

BACKGROUND ART

[0004] Known sensor assemblies typically comprise composite ceramic/metal
components that are brazed together using conventional brazing
techniques. Such a known sensor assembly might include a metal housing
with a metallised aluminium oxide bush brazed into the inner diameter of
the housing. A sensor body is then brazed into the internal diameter of
the bush.

[0005] The sensor body can be made of one or more layers of metal,
electrically conductive ceramic, electrically non-conductive ceramic that
is made conductive by having a layer of conductive material (e.g. a
metal) deposited on its surface, or a conductive ceramic/metal composite,
for example. Conductive layers can define electrodes or other sensing
elements or shield layers. Non-conductive layers can define insulating
spacers that are positioned between conductive layers. The layers that
form the sensor body can be machined as a preformed part and then bonded
to an adjacent layer or deposited on an adjacent layer using any suitable
deposition technique. If the outer layer of the sensor body is made
substantially from a ceramic material then its outer surface can be
metallised so that the sensor body can be brazed directly into the
housing using conventional brazing techniques without the need for the
intermediate bush.

[0006] The metal housing parts of the sensor assembly might be
manufactured from a low expansion alloy which is specifically designed to
have a coefficient of thermal expansion substantially similar to that of
the bush and/or the sensor body. If the sensor assembly is exposed to
high temperatures during operation then the housing, bush and sensor body
all expand at similar rates to minimise the thermal stress between the
individual components.

[0007] One problem with the use of low expansion alloys is that they tend
to oxidise at temperatures approaching 500° C. This places an
upper limit on the operating temperature of the sensor assembly. It can
be difficult to find a metal that is suitable for use at higher
temperatures and which also has a thermal expansion coefficient that is
substantially similar to that of the bush and/or the sensor body. A known
solution is to use so-called "active braze" techniques which allow
certain ceramic materials to be brazed to metals without the need for
metallised coatings and also provide a degree of compliance between the
two different materials to accommodate the different rates of thermal
expansion. In practice, however, the operating temperature of active
braze alloys is limited to about 800° C. which is still not
sufficiently high for certain operations. The compliant coatings that are
needed to provide the degree of compliance also tend to oxidise at
temperatures below 500° C. and it is normally necessary to provide
a hermetic seal at the braze interface to minimise the oxidation effect
when the operating temperature falls below this threshold.

[0008] Further problems are known to exist in situations where large
relative movements occur between the component parts of the sensor
assembly as a result of thermal expansion. Large relative movement can
only be accommodated by increasing the thickness of the complaint
coatings and this can place practical limitations on the design of the
sensor assembly.

SUMMARY OF THE INVENTION

[0009] The present invention provides a sensor assembly comprising: a
sensor body having a radial flange; and a housing having an annular
groove in which the radial flange of the sensor body is received, the
annular groove being defined by a pair of facing shoulders each having an
annular surface and a substantially cylindrical surface; wherein the
annular surfaces of the shoulders are in sliding contact with annular
surfaces of the flange and apply a compressive load to the flange.

[0010] The sensor body is not physically secured to the housing (e.g. by
brazing) but is firmly held within the housing as a result of the
compressive load that is applied to the flange by the annular surfaces of
the shoulders. The particular construction of the sensor assembly means
that there are no significant problems with differential thermal
expansion and the sensor assembly is therefore inherently suitable for
high temperature operation. The sensor assembly can be manufactured in a
cost-effective manner using conventional brazing techniques as described
in more detail below.

[0011] The sensor body is preferably formed substantially from a ceramic
material and can include one or more electrically conductive layers and
one or more electrically non-conductive layers, for example. The precise
shape and construction of the sensor body is not a critical feature of
the present invention and will depend on the type of sensor assembly. The
sensor body must, however, include the radial flange.

[0012] The housing is preferably a two-part housing formed from a high
temperature metal. More particularly, one of the facing shoulders is
preferably formed in a first housing part and the other of the facing
shoulders is preferably formed in a second housing part. The first and
second housing parts are secured together to form the two-part housing in
such a way that the shoulders are in register and define the annular
groove into which the radial flange of the sensor body is received. The
flange is therefore normally held between the two housing parts by the
applied compressive load.

[0013] The first housing part is preferably brazed to the second housing
part by a braze material. Any suitable braze material can be used.

[0014] The sliding contact between the housing and the annular surfaces of
the flange under the compressive load preferably provides a hermetic seal
between the housing and the sensor body. The hermetic seal is maintained
even when the sensor assembly is exposed to high operating temperatures.

[0015] The present invention provides a method of manufacturing a sensor
assembly comprising the steps of: providing a sensor body having a radial
flange; locating the sensor body in a two-part housing having an annular
groove in which the radial flange of the sensor body is received, the
annular groove being defined by a pair of facing shoulders each having an
annular surface and a substantially cylindrical surface, one shoulder
being formed in a first housing part and another shoulder being formed in
a second housing part; bringing the annular surfaces of the shoulders
into contact with the annular surfaces of the flange; and brazing the
first and second housing parts together to form an integral two-part
housing by (i) raising the sensor assembly to a particular brazing
temperature during which the first and second housing parts undergo
thermal expansion, (ii) applying a braze material to the first and second
housing parts in a molten state, and (iii) reducing the temperature of
the sensor assembly so that the braze material solidifies to secure the
first and second housing parts together to form the integral two-part
housing and during which the first and second housing parts undergo
thermal contraction to apply a compressive load to the flange.

[0016] In a preferred method the first and second housing parts are
assembled together to substantially surround the sensor body with
respective brazing surfaces in contact or in close proximity. During the
brazing process, as the sensor assembly is raised to a particular brazing
temperature, the first and second housing parts are preferably loaded to
maintain direct contact between the annular surfaces of the facing
shoulders and the annular surfaces of the flange. More particularly, the
annular surfaces of the shoulders are preferably brought into contact
with the annular surfaces of the flange by applying a loading which
forces the first and second housing parts towards each other in the axial
direction. At the particular brazing temperature, the braze material is
in the molten state and the contact between the first and second housing
parts and the flange is preferably maintained under load. The brazing
material is applied between the brazing surfaces of the first and second
housing parts. The braze material is typically applied when the sensor
assembly is at an ambient temperature (i.e. in "cold" application
process) so that it transitions to the molten state when the temperature
of the sensor assembly reaches the particular brazing temperature, but
the braze material can also be applied once the temperature of the sensor
assembly has reached the brazing temperature (i.e. in a "hot" application
process). As the temperature of the sensor assembly is subsequently
reduced, the braze material solidifies to fixedly secure the first and
second housing parts together to form an integral two-part housing
surrounding the sensor body, which is typically made substantially of
ceramic material. The first and second housing parts undergo thermal
contraction and effectively shrink onto the flange of the sensor body to
apply a significant compressive load onto the flange in the axial
direction. In other words, the housing contracts more than the sensor
body as the temperature decreases. The application of the compressive
load results in the creation of a hermetic seal between the housing and
the sensor body. Providing a hermetic seal is important because it
prevents moisture from penetrating the sensor assembly and reducing its
operational performance.

[0017] It will be readily appreciated that the compressive load that
arises from the shrinkage of the first and second housing parts is
different to the external loading that is applied during the brazing
process and is maintained throughout the operating lifetime of the sensor
assembly. Ceramic materials are known to cope well with compressive loads
and assessment shows that the risk of damage to the sensor body during
the brazing process is very low. In practice the compressive load applied
when the sensor assembly is at a high operating temperature will be
slightly less than for ambient temperature because of the differential
thermal expansion between the housing and the sensor body in the axial
direction. However, the compressive load will always be at a sufficient
level to maintain the hermetic seal.

[0018] When the sensor assembly is used at a high operating temperature
the housing undergoes thermal expansion and expands away from the sensor
body in the radial direction. In other words, the housing expands more
than the sensor body as the temperature increases. The expansion causes
the annular surfaces of the housing to slide relative to the annular
surfaces of the flange in the radial direction and this sliding contact
may be promoted by a suitable choice of material for the sensor body (or
its contact surfaces) and/or the housing. Any movement of the housing
relative to the sensor body in the axial direction is very small
(typically in the order of a few microns) and is accommodated by the
material properties of the housing.

[0019] The maximum operating temperature of the sensor assembly is
effectively limited by the brazing temperature, taking into account the
mechanical properties of the braze material and the housing material etc.
It is expected that for typical brazing temperatures in excess of
1200° C. then the sensor assembly can function properly at
temperatures approaching 1000° C. at the brazing surfaces. It will
be readily appreciated that the temperature at other parts of the sensor
assembly removed from the brazing surfaces may be significantly higher.
For example, in the case of a capacitive sensor that is used to measure
the clearance between the tip of a gas turbine engine blade and the
surrounding casing then the front face of the sensor assembly might be
exposed to temperatures of about 1500° C. with a cooler
temperature being experienced at the rear of the sensor assembly where
the brazing surfaces are located.

DRAWINGS

[0020] FIG. 1 is an exploded cross section diagram showing a sensor
assembly according to the preset invention;

[0022] FIGS. 3A and 3B are cross section diagrams showing the brazing
process by which the two parts of the housing of the sensor assembly are
secured together to surround the inner sensor body; and

[0023] FIG. 4 is a cross section diagram showing the complete sensor
assembly of FIG. 2 at a high operating temperature;

[0024] FIGS. 1 and 2 show a sensor assembly with a sensor body 2 that is
made of ceramic material and an integral two-part metal housing 20.

[0025] The sensor body 2 includes an electrode 4 formed from electrically
conductive ceramic and an outer layer 6 formed from electrically
non-conductive ceramic which acts as an insulating layer. The outer layer
6 can be deposited on, or bonded to, the inner electrode 4 using any
suitable manufacturing technique such that the sensor body 2 is an
integral structure. The different ceramic materials that are used to form
the electrode 4 and the outer layer 6 can be selected to have
substantially similar thermal expansion coefficients. It will be readily
appreciated that the sensor body 2 may have any convenient or suitable
construction depending on the type of sensor with one or more layers of
metal, electrically conductive ceramic, electrically non-conductive
ceramic that is made conductive by having a layer of conductive material
(e.g. a metal) deposited on its surface, or a conductive ceramic/metal
composite, for example.

[0026] The sensor body 2 includes a flange 8 that protrudes radially
outwardly from the outer cylindrical surface 10 of the outer layer 6. The
flange 8 includes a first annular surface 12, a second annular surface 14
and a cylindrical surface 16.

[0027] The housing 20 is made up of a first housing part 22 and a second
housing part 24.

[0028] The first housing part 22 includes a central cylindrical bore 26
and an annular shoulder 28 having a larger diameter than the central bore
and which is defined by an annular surface 30 and a cylindrical surface
32.

[0029] The second housing part 24 includes a central cylindrical bore 34
and an annular shoulder 36 having a larger diameter than the central bore
and which is defined by an annular surface 38 and a cylindrical surface
40. An outer bore 42 has a larger diameter than the cylindrical surface
40 and is defined by an axially extending flange 44 of the second housing
part.

[0030] The central bores 26, 34 in each housing part are sized to receive
the sensor body 2 with a close tolerance fit to the outer surface 10 when
the sensor assembly is at an ambient temperature. However, the radial fit
between the outer surface of the sensor body flange and the inner surface
of the housing is generally not considered to be critical.

[0031] The first housing part 22 includes a cylindrical brazing surface
46. When the first housing part 22 is assembled to the second housing
part 24 as shown in FIG. 2 then the brazing surface 46 faces a
corresponding cylindrical brazing surface 48 of the flange 44. More
particularly, when assembled together, the brazing surface 46 of the
first housing part 22 is located radially within the flange 44 of the
second housing part 24 with a close tolerance fit. The annular shoulders
28, 36 are also aligned to define an annular groove or recess into which
the flange 8 of the sensor body 2 is received with a close tolerance fit
when the sensor assembly is at an ambient temperature. Although not
shown, the upper surfaces of the first and second housing parts 22, 24
may be chamfered adjacent the respective brazing surface so that they
when they are assembled together they define a narrow annular groove into
which the brazing material can be deposited.

[0032] The assembly steps of the sensor assembly will now be explained
with reference to FIG. 3A and 3B.

[0033] The second housing part 24 is supported in a suitable frame or
support S. The sensor body 2 is inserted into the central cylindrical
bore 34 of the second housing part 24 and the first housing part 22 is
then located to the second housing part 24 with the upper part of the
sensor body 2 positioned in the central cylindrical bore 26 and the
respective brazing surfaces 46, 48 axially aligned as shown in FIG. 3A.

[0034] The first and second housing parts 22, 24 are then secured together
by a brazing process. A brazing material BM (optionally in the form of a
paste) is applied to the upper surface of the sensor assembly at the
interface between the respective brazing surfaces 46, 48 of the first and
second housing parts 22, 24. The brazing material may sit in the narrow
annular groove (not shown) mentioned above. The sensor assembly is raised
to a particular brazing temperature that is determined by the brazing
material that it to be used. During the brazing process, an axial loading
is applied to the first housing part 22 (as indicated by the arrows) to
maintain a direct contact between the annular surfaces 30, 38 of the
facing shoulders and the annular surfaces 12, 14 of the flange 8.

[0035] As the temperature of the sensor assembly is raised to the brazing
temperature, the first and second housing parts 22, 24 expand away from
the sensor body 2 in the radial direction as shown in FIG. 3B. Expansion
of the first and second housing parts 22, 24 in the axial direction is
limited and direct contact between the annular surfaces 30, 38 of the
facing shoulders and the annular surfaces 12, 14 of the flange 8 is
maintained by the axial loading. The axial gap 50 ensures that there is
no direct contact between the first and second housing parts 22, 24 that
would otherwise limit or restrict the amount of axial loading that can be
applied to the flange 8. In other words, the axial compression force
acting on the annular surfaces 12, 14 of the flange 8 is determined
solely by the axial loading applied during the brazing process and the
subsequent compressive load.

[0036] Once the temperature of the sensor assembly reaches the brazing
temperature, the braze material is in the molten state and is drawn down
into the interface between the brazing surfaces 46, 48 of the first and
second housing parts 22, 24 by a capillary action.

[0037] As the temperature of the sensor assembly is subsequently reduced,
the braze material solidifies to secure the first and second housing
parts 22, 24 together to form an integral two-part housing 20 surrounding
the sensor body 2. More particularly, the first and second housing parts
22, 24 are fixedly secured together by the brazing material at the join
or interface between the facing brazing surfaces 46, 48. The first and
second housing parts 22, 24 are not secured together at any other
interface and are not secured in any way to the sensor body 2. The
absence of fixing between the housing 20 and the sensor body 2 means that
the sensor assembly does not experience any stresses as a result of
differential thermal expansion which might in other circumstances lead to
the disintegration or failure of the ceramic and/or metal components.

[0038] The first and second housing parts 22, 24 undergo thermal
contraction and effectively shrink onto the flange 8 of the sensor body 2
to apply a significant compressive load onto the flange in the axial
direction. The application of the compressive load during the brazing
process results in the creation of a hermetic seal between the housing 20
and the sensor body 2. More particularly, the hermetic seal is formed
between the annular surfaces 30, 38 of the facing shoulders and the
annular surfaces 12, 14 of the flange 8. One or more of the annular
surfaces may be machined, coated or otherwise treated to provide a smooth
surface finish so that close physical contact is established across as
large an area as possible.

[0039] When the sensor assembly is used at a high operating temperature
the first and second housing parts 22, 24 undergo thermal expansion and
expand away from the sensor body 2 in the radial direction as shown in
FIG. 4. The expansion causes the annular surfaces 30, 38 of the housing
to slide relative to the annular surfaces 12, 14 of the flange 8 in the
radial direction. However, the flange 8 of the sensor body 2 remains
under a compressive load at the high operating temperature and the
hermetic seal is maintained at all times during the operational lifetime
of the sensor assembly.

[0040] The thickness of the flange 8 in the axial direction is preferably
kept to a minimum in order to minimise the differential thermal expansion
between the flange and the first and second housing parts 22, 24. It will
be readily appreciated that if the differential thermal expansion is too
large then this might result in the hermetic seal being compromised.
However, the flange 8 must also be thick enough to cope with the external
loading that is applied during the brazing process and the resulting
compression load. The flange 8 also preferably protrudes beyond the outer
surface of the sensor body 2 by an amount that will allow radial
expansion of the first and second housing parts 22, 24 away from the
sensor body while keeping sufficient contact between the respective
annular surfaces to maintain the hermetic seal.